1. Field of the Invention
This invention relates to photoresist compositions particularly suitable for DUV exposure. The resists are capable of forming uniform features of submicron dimension across the full width of a semiconductor wafer. More particularly, the invention provides a photoresist film where linewidth variation as a function of high temperature post exposure bake is minimized.
2. Background
Photoresists are photosensitive films for transfer of an image to a substrate. These resists form negative or positive images. After coating a photoresist coating composition onto a substrate, the coating is exposed through a patterned photomask to a source of activating energy such as ultraviolet light to form a latent image in the coating. The photomask has areas both opaque and transparent to activating radiation that define a desired image to be transferred to the underlying substrate. A relief image is provided by development of the latent image pattern in the resist coating.
The use of photoresists is generally described, for example, by DeForest, Photoresist Materials and Processes, McGraw Hill Book Company, New York (1975), and by Moreau, Semiconductor Lithography, Principals, Practices and Materials, Plenum Press, New York (1988).
Recent developments in photoresist imaging involve formulation of photoresists imaged by exposure of coatings to deep ultraviolet (DUV) radiation. As is known by those in the art, DUV refers to exposure radiation having a wavelength in the range of about 350 nm or less, more typically in the range of about 300 nm or less and most often, 248 nm. Photoresists imaged by DUV exposure offer the advantage of providing patterns of reduced feature size compared to photoresists imaged by exposure to radiation of longer wavelength.
"Chemically amplified" photoresist compositions have been developed that are especially suitable for DUV imaging. Chemically amplified photoresists may be negative or positive-acting and rely on many crosslinking events (in the case of a negative-acting resist) or deprotection reactions (in the case of a positive-acting resist), each catalyzed by photogenerated acid or base. In the case of the positive chemically amplified resist, certain cationic photoinitiators capable of yielding a photogenerated acid have been used to induce cleavage of certain "blocking" groups pendant from a photoresist binder, or cleavage of certain groups that comprise a certain photoresist binder backbone. See, for example, U.S. Pat. Nos. 4,491,628; 4,810,613; 4,883,740; 4,968,581; 5,075,199; 5,258,257; 5,362,600; 5,492,273; 5,558,971 and Canadian Patent Application No. 2,001,384, each incorporated herein by reference for its disclosure of DUV positive working chemically amplified photoresist formulations and blocking groups. Upon exposure of a photoresist coating and a post exposure bake, selected cleavage of the blocking group results in formation of a polar functional group, e.g., hydroxyl, carboxyl or imide. The generation of a polar functional group provides differential solubility characteristics between exposed and unexposed areas of the resist coating.
The above patents illustrate a variety of blocking groups that may be utilized for positive working chemically amplified photoresists. Each blocking group requires a given quantity of energy to effect deblocking. The required energy is known in the art as the activation energy. A means to determine activation energy is described by Wallraff et al., Kinetics of Chemically Amplified Resists, Photopolymers Principles, Processes, and Materials, Tenth International Technical Conference, pp. 11-17, Oct. 31-Nov. 2, 1994, Society of Plastic Engineers, Inc. and by Wallraff et al., J. Vac. Sci Technol., 1995, 12 (6) 3857. Activation energy is expressed in units of Kcal/mol. Blocking groups having greater activation energy for deblocking require more severe conditions to effect deblocking. Means for overcoming greater activation energy include use of a stronger photogenerated acid and/or higher bake temperatures.
Many different blocking groups are disclosed in the above identified patents. For example, in U.S. Pat. No. 5,558,971, the blocking group is an acetal or ketal group of the formula "--OCR.sup.1 R.sup.2 OR.sup.3 " where R.sup.1 and R.sup.2 are independently a hydrogen atom, a straight-chain, branched or cyclic alkyl group having 1-6 carbon atoms, a straight-chain or branched haloalkyl group having 1-6 carbon atoms, or a phenyl group, provided that R.sup.1 and R.sup.2 are not hydrogen at the same time, or R.sup.1 and R.sup.2 may combine to form a methylene chain having 2-5 carbon atoms, and R.sup.3 is a straight chain, branched or cyclic alkyl group having 1-10 carbon atoms, a straight-chain, branched or cyclic haloalkyl group having 1-6 carbon atoms, an acetyl group or an aralkyl group.
The acetal or ketal group as represented by the above formula is deblocked at a relatively low activation energy, typically from about 10 to 20 Kcal/mol. To effect deblocking, a relatively weak photogenerated acid and/or a relatively low temperature post exposure bake or both may be used to effect the deblocking reaction. Though this is desirable for processing of the photoresist coating, a low activation energy formulation suffers certain disadvantages. For example, deblocking of the blocking group may randomly occur during storage of the photoresist in its container. A decreased number of blocking groups on the polymer backbone would likely result in an unpredictable change in resist photospeed upon imaging.
To avoid storage instability, certain vendors of chemically amplified resists have used blocking groups that require a greater activation energy. For example, in U.S. Pat. No. 5,362,600, the blocking group conforms to the formula --CR.sup.4 R.sup.5 C(.dbd.O)OR.sup.6 where each of R.sup.4 and R.sup.5 is independently selected from the hydrogen, an electron withdrawing group such as halogen, lower alkyl having 1 to about 10 carbon atoms, and substituted lower alkyl having 1 to about 10 carbon atoms; and R.sup.6 is a substituted or unsubstituted lower alkyl having from 1 to about 10 carbon atoms, substituted or unsubstituted aryl having from 1 to about 10 carbon atoms, and substituted or unsubstituted benzyl having 7 to about 10 carbon atoms. The substituents can be, for example, one or more of halogen, lower alkyl, lower alkoxy, aryl or benzyl. R.sup.4 and R.sup.5 desirably are each hydrogen. If R.sup.4 and/or R.sup.5 are halogen or other electron-withdrawing group, upon acidic cleavage of the acetate group, a highly polar group is formed providing enhanced solubility differential between exposed and unexposed regions of the photoresist coating. The difluoro group (i.e., R.sup.4 and R.sup.5 are both fluoro) is especially suitable and provides extremely high dissolution differential between exposed and unexposed regions with only modest levels of substitution of hydroxy groups on the polymer binder.
For the high energy blocking groups described above, an activation energy of at least 20 Kcal/mole is required and typically, the required activation energy is within the range of from 25 to 40 Kcal/mole. To enable deblocking to occur, it is necessary to use one or both of a photoacid generator capable of liberating a strong acid and/or a high temperature post exposure bake, typically a temperature in excess of 120.degree. C. and preferably, a temperature of from about 130.degree. C. to 150.degree. C. or higher.
For reasons set forth above, those photoresists using blocking groups requiring high activation energy are generally subjected to one and often two high temperature baking steps. In practice, it has been found that with high temperature baking, minor variations in the bake temperature, i.e., variations of .+-.1.degree. C., across the width of the photoresist coating can lead to significant variation in linewidth across the developed coating and that this variation increases with increased bake temperature. This sensitivity is referred to in the art as PEB sensitivity which is defined as changes in linewidth at a fixed exposure dose on wafers that are post-exposure baked at increasing temperatures. The measured linewidth on each wafer is plotted against the PEB temperature and the PEB sensitivity in nm/.degree.C. is the slope of the line. PEB sensitivity may be as much as 5% per degree Celsius. It is known that it is difficult to maintain a uniform temperature across the full width of the resist coating--i.e., across the full width of a wafer coated with photoresist which may be 8 or more inches in diameter.
Linewidth variation is unacceptable for most commercial applications. Therefore, it would be desirable to have chemically amplified photoresist compositions capable of providing highly resolved fine line images, including images of submicron and sub half-micron dimension, which are PEB insensitive. It would be particularly desirable to have such a chemically amplified photoresist where variation in linewidth as a function of post exposure bake temperature is reduced or eliminated.